Propellant compositions are useful for a variety of applications. One such application is in vehicle air bag restraint devices. In such restraint devices, it is important to reduce the toxicity of gases produced upon combustion of the propellant. It is also desirable that the propellant composition burn in a smokeless or nearly smokeless fashion because the presence of smoke can cause various problems. For example, after an accident in which an air bag has been deployed, smoke not only hinders visibility, it also interferes with any ongoing rescue efforts. Thus, it is desirable that propellant composition combustion products be smoke-free or nearly so.
Another application of propellant compositions is their use in rockets and in other munitions as propulsive propellant compositions. Combustion of propulsive propellant compositions in rockets and the like provides the energy required to transport them over long distances towards a given target. During battle, it is critical to maintain the advantage of surprise and stealth. Therefore, it is desirable that rockets powered by propulsive propellant compositions be as undetectable as possible upon launch and during deployment.
To maintain the advantages of stealth and surprise, it is important that the propellant composition be smoke-free or nearly so during combustion. In an effort to meet the requirement of a smoke-free combustible propellant composition, several compositions have been developed by the U.S. military. Among the compositions developed are the "double base" propellant compositions. As is known in the art, "double base" refers to a propellant composition containing both nitroglycerine (NG) and nitrocellulose (NC). Double base propellants are prone to premature explosion or premature deflagration in response to various unplanned stimuli (e.g., fire, heat, shrapnel, bullets, other fragments etc.) that may be encountered in battle. In addition, for propulsive applications, the energy output upon combustion of double base propellants is sometimes insufficient. Thus, the addition of energetic additives such as cyclotetramethylene tetranitramine (HMX) and/or cyclotrimethylene trinitramine (RDX) is often required to provide the energy output sought during combustion. However, the addition of such energetic additives exacerbates the already hazardous tendency of double base propellants to premature explosion or premature deflagration.
Nevertheless, to fulfill the smoke free and high energy requirements of propulsive propellants, as defined herein, propellant compositions including double base propellants were pursued at the expense of safety, especially in regards to naval operations. Consequently, the U.S. Navy has taken the lead in formulating a series of standards concerning insensitive ammunition requirements, formalized as MIL-STD-2105B, incorporated herein by reference in its entirety. Equivalent insensitive ammunition standards have been adopted by most major military powers (e.g., England, France, Germany, etc.). These standards require that propellant compositions meet or exceed insensitive ammunition safety standards for the weapons platforms for which they were designed.
Further, with regards to military propulsive applications, various smoke characteristics required of propellant compositions have been strictly defined. Based on the empirical work performed by the U.S. Missile Command at Redstone Arsenal and some of their counterparts in other countries, industry accepted definitions of "minimum smoke" and "reduced smoke" have been promulgated in STANAG 6016 (NATO Standardized Agreement Solid Propellant Smoke Classification). STANAG 6016 is incorporated herein by reference in its entirety. The smoke effluent is calculated by a number of thermo-chemical codes that are well known in the industry. For example, STANAG 6016 classifications "AA" and "AC" correspond to the definitions of minimum smoke and reduced smoke, respectively. The "smoke-free", "nearly smoke free" and/or "substantially smoke free" terms as used herein are synonymous with the definition of minimum smoke (i.e., code AA).
To meet these requirements (i.e., smoke free--minimum smoke in accordance with STANAG 6016; high energy output and safety--in accordance with Insensitive Ammunitions Requirements formalized as MIL-STD-2105B) attempts have been made to develop non-double base propellant compositions that are smoke free, yet safe for handling. For example, ammonium nitrate, metal nitrate, alkali earth metal nitrate, ammonium perchlorate and metal perchlorate propellant compositions and the like have been used. However, these propellant compositions present several problems. Metal nitrates, typically, produce solid particles upon combustion. These solid particles form a visible smoke referred to as "primary smoke" which is undesirable. Ammonium or metal perchlorates produce hydrogen chloride during combustion. Hydrogen chloride reacts with moisture in the ambient air to yield a liquid/gas aerosol. The aerosol forms another visible smoke referred to as "secondary smoke". Either "primary smoke" or "secondary smoke" formed as an effluent from the combustion of a propulsive propellant composition negates the advantage of surprise. The smoke trail aids opposing forces in destroying or otherwise countering the incoming missile. In addition, such effluent smoke points to the launch position. During battle, such smoke places launch personnel in greater danger of potentially successful retaliation, e.g., by counter battery fire.
Ammonium nitrate as a propellant ingredient may produce a propellant that does not produce primary or secondary smoke upon combustion. However, ammonium nitrate presents other drawbacks as a propellant component. Principally, it is recognized that ammonium nitrate undergoes several crystal phase changes at various well-recognized temperatures. Pure ammonium nitrate undergoes a series of structural and volumetric crystal phase transformations over typical operating temperature ranges. In pure ammonium nitrate, structural crystal phase transitions are observed at about -18.degree. C., 32.3.degree. C., 84.2.degree. C. and 125.2.degree. C., respectively. The phase transition at about 32.3.degree. C. is particularly troublesome. A large volumetric change (about 3.7%) in the crystal phase of ammonium nitrate is observed when the temperature cycles above and below about 32.3.degree. C. (i.e., transition between phase IV (below 32.3.degree. C.) and phase III (above 32.3.degree. C.)). As the ammonium nitrate cycles between phase IV and phase III, it expands and contracts. Repeated cycling through the phase IV to phase III transition temperature (i.e., about 32.3.degree. C.) is associated with ammonium nitrate grain growth and destruction of grain integrity. The result is that there is porosity and loss in mechanical strength of ammonium nitrate based propellant compositions.
As used herein, the term "age-stabilized" refers to a state of ammonium nitrate wherein the crystal phase III-IV and volumetric changes associated with thermal cycling are substantially reduced. Thus, the shelf-life of an ammonium nitrate propellant composition is considerably increased from about 1-2 years to about 5-20 years or more.
Further, the term "strengthened", as used herein, refers to a state of ammonium nitrate propellant wherein the tensile strength of the propellant is increased without unduly sacrificing elongation or, alternatively, is accompanied by an increase in elongation. The strengthened ammonium nitrate propellant composition is substantially resistant to physical destruction of the propellant.
As also used herein, the term "safe" refers to an ammonium nitrate propellant composition that meets or exceeds the insensitive ammunition requirements promulgated in MIL-STD-2105B wherein the tendency to violent deflagration or explosion is substantially reduced and the shelf-life is substantially increased from about 1-2 years to about 5-20 years or more. Further, the term "safe" is used herein to refer to an ammonium nitrate propellant composition wherein the tendency to form grain fissures due to crystal phase changes is substantially reduced or altogether eliminated.
It is feared that non-strengthened/non-age-stabilized ammonium nitrate propellant compositions that have been stored (e.g., either in munitions or in vehicle air bag restraint devices) for more than about 1 to 2 years may have undergone several crystal phase changes to the extent that the physical integrity of the propellant has been compromised and the propellant will no longer perform in the desired manner. Consequently, the useful shelf-life of prior art ammonium nitrate propellant compositions is disadvantageously shortened. Thus, it is desirable to formulate a smoke-free (or substantially smoke free) yet safe ammonium nitrate propellant composition having an extended shelf-life.
Typically, a propulsive or gas generating device containing a propellant composition requires a shelf-life from about 5 to about 20 years or more. The shelf-life of the device is largely dependent on the shelf-life of the propellant composition contained therein. Typically, a desirable shelf-life for a munition (propulsive) propellant composition or a vehicle air bag (gas producing) propellant composition is about 5 or more years, preferably, from about 7 to 20 years. In order to obtain longer shelf-life ammonium nitrate propellant compositions, efforts have been directed at solving the crystal phase stabilization problem (i.e., of ammonium nitrate). For example, various patents and publications suggest the use of KNO.sub.3, KF, metal dinitramide, or metal oxides such as MgO, NiO, CuO and/or ZnO as additives that yield phase stabilized ammonium nitrate. See, for example U.S. Pat. No. 4,158,583 to Anderson; U.S. Pat. No. 5,076,868 to Doll et al.; U.S. Pat. No. 5,271,778 to Bradford et al.; U.S. Pat. No. 5,292,387 to Highsmith et al; and U.S. Pat. No. 4,552,736 to Mishra; U.S. Pat. No. 5,545,272 to Poole et al. See also, Choi, C. S., and Prask, H. J., Phase Transitions in Ammonium Nitrate, J. Appl. Cryst., Vol. 13, pp. 403-409 (1980).
However, various problems are associated with the use of the aforementioned phase stabilizing additives. For example, the use of potassium nitrate leads to the formation of large amounts of undesirable residue as combustion products. See U.S. Pat. No. 4,552,736 to Mishra. When KF is used, it must be added to the molten phase (I) of ammonium nitrate. Thereafter, the KF modified ammonium nitrate is cooled. The requirement for melting ammonium nitrate before adding KF is cumbersome, expensive and time consuming. In addition, the effluent of a device using such a propellant is corrosive, smoky (with an enhanced radar cross section) and toxic.
The use of the metal oxides also has several drawbacks. For example, solid particulates are formed upon combustion when MgO, Nio, CuO and/or ZnO are used. Solid particulates, as previously noted, contribute to the formation of primary smoke which is undesirable. Additionally, NiO is carcinogenic. Further, NiO and CuO present environmental hazards. In addition, both Nio and ZnO are only marginally effective. That is, once exposed to moisture, these oxides are no longer effective ammonium nitrate phase stabilizers. Further, NiO and ZnO increase the detonatability of the ammonium nitrate which is undesirable. Additionally, manufacturing propellant compositions including NiO and/or ZnO is more expensive. Similarly, the use of metal dinitramides (see '387 to Highsmith et al.) also leads to the formation of primary smoke upon combustion. Thus, none of the known ammonium nitrate phase stabilizers are entirely satisfactory for forming a safe, age-stabilized and smoke-free ammonium nitrate propellant composition having a long shelf-life.
The occurrence of phase III in ammonium nitrate depends on the presence of water, e.g., down to as little as about 0.1% by weight of the ammonium nitrate. See Choi et al., J. Appl. Cryst., Vol. 13, p. 403 (1980). In particular, according to the '736 patent to Mishra, supra, (at column 2, lines 66-68), a high moisture content is said to favor III-IV phase transitions. Further, according to U.S. Pat. No. 4,486,396 to Kjohl et al., (at column 1, lines 30-32), these phase transitions render the ammonium nitrate less stable to thermal cycling.
U.S. Pat. No. 5,061,511 to Baczuk ('511) suggests the use of aluminum silicate molecular sieves (having a pore size of less than about 10 angstroms) as a stabilizer in propellant compositions such as single base or double base propellant. In particular, the '511 patent is directed to propellant compositions that give off gases during the aging process. These propellant compositions include nitrocellulose and nitroglycerin, high energy fluorine containing propellants, single or double base nitrate ester propellants and composite propellants such as ammonium perchlorate/Al with rubber binders. The undesirable gases given off by these propellants during aging include N.sub.2, CO.sub.2, CO, NO.sub.x, and F.sub.2. Likewise, U.S. Pat. No. 4,045,261 to Baczuk ('261) suggests the use of a molecular sieve (having a pore size of 10 angstroms or more) as part of a stabilization system for a urethane cross-linked double base propellant composition to scavenge nitric acid.
Since ammonium nitrate is not a propellant of the class described by Baczuk (i.e., See '511) and it does not give off the N.sub.2, CO.sub.2, CO, NO.sub.x or F.sub.2 gases (i.e., see '511) during aging, there is no expectation that molecular sieves in general, much less those having a pore size of 10 angstroms or less would stabilize ammonium nitrate. Similarly, since ammonium nitrate is not a urethane cross-linked double base propellant (i.e., see '261), there is, likewise, no expectation that molecular sieves, e.g., having a pore size of 10 angstroms or more, would stabilize ammonium nitrate against volumetric crystal phase changes.
Nevertheless, since water is associated with the undesirable crystal phase changes of ammonium nitrate, Kjohl et al., supra, used porous additives which could absorb water to stabilize ammonium nitrate. They further discovered that the presence of water absorbing porous particles resulted in no movement of water in the ammonium nitrate particles and that, during thermal cycling, swelling of ammonium nitrate was observed only to a small extent. Kjohl et al., however, state that the porous particles should be added to the ammonium nitrate after the ammonium nitrate is dried. Finally, they state that not any type of porous particle is suitable for stabilizing ammonium nitrate. For example, according to Kjohl et al., silicates of the molecular sieve type can bind water, but it has been found difficult to give such particles the required particle size and binding to the ammonium nitrate particles. In effect, Kjohl et al. conclude that molecular sieves performed poorly in stabilizing ammonium nitrates (see column 3, lines 18-23).
Thus, there is still an existing need to provide a safe, age-stabilized and/or strengthened ammonium nitrate propellant composition having a long shelf-life that is substantially smoke free upon combustion as well as a method for making the same.